This spring, I ran 13 miles wearing a free T-shirt from a cement supplier, a pair of leggings I got from Oxfam, a £4 sports bra from Walthamstow market and a pair of £15 trainers off Vinted. I ran it, I am sure, at exactly the same speed and with the same level of comfort as I would have in a pair of £150 designer running tights and an £86 running shirt designed by Paul McCartney’s daughter. Which is to say that I hoofed along, sweating and sniffing and occasionally stopping to wee in a bush.

Fifteen years after taking up regular running, and at a point where I run six miles at least three times a week, I still refuse to pay more than £20 for any so-called “sportswear”. Sportswear, as far as I’m concerned, is whatever you wear when you’re doing sports. And if that includes old pyjamas, holiday shorts and a pair of trainers left over after a birthday party, then so be it.

And yet, as my body swells and melts like a burrata in a delicatessen window, I am becoming increasingly aware that people around me are spending significant sums on their gear. A decent sports bra or a pair of trainers designed to suit your gait, say. Do they work? Is it a good investment? Am I being wilfully dismissive of these emperors’ new clothes?

“We had a member turn up at the gym in a dress one time,” says Ajaye Hunn-Phillips, a personal trainer and co-founder of Oxford-based inclusive gym The Project PT. “It was a hot day, she was going on to something afterwards, so she just came in and trained in a dress. After all, if a tennis skirt is considered fine, then why not a dress?”

This is an attitude I can get behind. But can it really be so simple as to just wear what you feel comfortable in? “Of course, there are definitely pieces of equipment that are important to certain sports and certain fitness practices,” says Hunn-Phillips, who founded the gym after 20 years in the fitness industry. “But on the whole, why put a barrier there in the first place? We did a research project on physical activity for teenage girls and one of the big barriers was clothing – having to get changed and then not feeling comfortable in that PE kit.”

As a result, The Project PT has no rules about what its users wear. “It was a big thing for us in setting up the gym: wear what you want – it doesn’t matter. There are a lot of places that don’t allow jeans or barefoot training because they’re steeped in this historical precedent of gyms being set up to serve people who were affluent, who can afford another set of clothes. Like golf clubs insisting you own a separate sports jacket. But there’s absolutely no reason for it.”

Feeling invigorated and heartened by Hunn-Phillips’s attitude, I head out for a quick run. As I plan to stop halfway for a swim, I decide to run in my swimming costume. Is this just small-breast privilege, I wonder, or are sports bras also a swizz?

“Sports bras might just exist to make everyone comfortable except the person running in them,” says Casey Johnston, a Californian writer and fitness influencer, whose She’s a Beast newsletter essay on running braless has provoked interest and surprise in equal measure. Many women consider it absolutely essential – non-negotiable - to wear a sports bra to run. “I have pretty large breasts,” says Johnston, “but running without a bra was vastly more comfortable than I expected. If you’re not running really fast with a lot of frenetic motion, a gentle jog just makes large tatas just sort of roll back and forth, which is a motion they can’t accomplish in a sports bra. They do still move around a lot and very noticeably, but who is that a problem for, exactly? Not the person running.”

After her years of strength training and other exercise, what would Johnston say to someone who wants to exercise but is put off by the kinds of bodies they see in shops and adverts? “It’s a hard lesson to absorb, but advertising is not real,” she replies. “For me, exercise is a huge part of managing how I feel in my body. I hurt if I sit around too much, I don’t sleep well, I feel better if I spend time outside. These are basic human rights! Exercising does not have to be a betrayal of some grand, embarrassing ambition to become one of the hot people you see in ads. Feeling good in the only body you have, the body you are literally obliged to be in, is something you are entitled to, and physical activity is part of that for a lot of people.”

Flapping home from my run, smelling of river silt and sweat, I look down at my secondhand Brooks trainers and car-boot-sale shorts. In a previous life, working for fashion magazines and hipster websites, I was made acutely aware of the sort of money – not to mention time and energy – being ploughed into collecting this kind of gear by my colleagues, male and female alike. But is the hunt for high-performance sportswear really just another way of marking income and status?

“I think athletic wear brands – actually all fashion brands to an extent, but especially athletic ones – profit from arrival syndrome, ie the idea that once you have a certain item or look a certain way, you’ll be valid and acceptable,” says Lauren Bravo, author of How to Break Up With Fast Fashion. “But we’re kidding ourselves if we think we ‘need’ a whole wardrobe of specialist leggings and cropped tops to work out. Social media has played its part in making fitness an outward performance, whereby we feel like outcasts if we’re the only ones at pilates without designer Lycra.”

Then there is the question of materials. “While a cotton T-shirt would be perfectly fine a lot of the time, most trendy athletic gear is made from synthetics like Lycra – which means they’re oil-derived, and don’t break down in landfill,” says Bravo. “Polyester sheds microplastics into our waterways; it can take up to 200 years to biodegrade – and because it hasn’t even been around for 200 years yet, we don’t honestly know yet what the environmental impact of that will be.” Some brands now claim to be using recycled polyester but, as Bravo points out, “That doesn’t make much difference if we’re going to cast them off into landfill after a year or less.”

It’s all very well for me, a middle-aged, freelance writer to take this laissez-faire attitude to sportswear, you might think. For me, exercise is just a hobby – a way of keeping relatively healthy physically and mentally, with the luxury of doing so outside and frequently alone. But what about people whose careers are reliant on their bodies, fitness and movement?

“I am a firm believer that you can and should wear whatever you like to do yoga,” says Emma-Jane Greig, a yoga teacher and founder of the dance company Body Politic. “Granted, some clothing brands make ‘better’ leggings – and by better I mean less see-through – but these are generally linked to eye-watering price tags and are in no way a reflection on whether or not you can and should do yoga.”

And yet, argues Greig, things are less simple when it comes to the world of dance. Her hip-hop dance company aims to empower young people from underrepresented communities on and off the stage. “Dance is built on the notion of performance and expression, so there is – and perhaps always will be – this pressure that you also need to look the part,” says Greig. “But I think there is something important about how we create safe and welcoming spaces for young people to dance and feel comfortable in their own bodies and clothing. A lot of Body Politic’s work takes place in community settings (without huge mirrors and LED strip lighting), and I truly believe that this makes a difference. Young people are forced to move from a place of connection and expression rather than ogling themselves in the mirror or comparing themselves with others.”

As I peel off my socks, stolen from my husband’s Christmas stash, I take a second to look at my body in the bathroom mirror. It is, in every sense, unremarkable, but I am probably more comfortable in it today than I have ever been before, for the simple reason that I move it, to the point of sweating, every day. Does that habit necessitate hundreds of pounds’ worth of high performance sportswear? Not on your nelly. As Lauren Bravo puts it: “Far better to take the opportunity to wear all those hen party T-shirts and corporate team-building T-shirts you’ve amassed over the years. Let’s give them their moment in the sun.”